In 'commit 752f66a75aba ("bcache: use REQ_PRIO to indicate bio for
metadata")' REQ_META is replaced by REQ_PRIO to indicate metadata bio.
This assumption is not always correct, e.g. XFS uses REQ_META to mark
metadata bio other than REQ_PRIO. This is why Nix noticed that bcache
does not cache metadata for XFS after the above commit.
Thanks to Dave Chinner, he explains the difference between REQ_META and
REQ_PRIO from view of file system developer. Here I quote part of his
explanation from mailing list,
REQ_META is used for metadata. REQ_PRIO is used to communicate to
the lower layers that the submitter considers this IO to be more
important that non REQ_PRIO IO and so dispatch should be expedited.
IOWs, if the filesystem considers metadata IO to be more important
that user data IO, then it will use REQ_PRIO | REQ_META rather than
just REQ_META.
Then it seems bios with REQ_META or REQ_PRIO should both be cached for
performance optimation, because they are all probably low I/O latency
demand by upper layer (e.g. file system).
So in this patch, when we want to decide whether to bypass the cache,
REQ_META and REQ_PRIO are both checked. Then both metadata and
high priority I/O requests will be handled properly.
Reported-by: Nix <nix@xxxxxxxxxxxxx>
Signed-off-by: Coly Li <colyli@xxxxxxx>
Reviewed-by: Andre Noll <maan@xxxxxxxxxxxxxxxx>
Tested-by: Nix <nix@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
---
drivers/md/bcache/request.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 15070412a32e..f101bfe8657a 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -392,10 +392,11 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
/*
* Flag for bypass if the IO is for read-ahead or background,
- * unless the read-ahead request is for metadata (eg, for gfs2).
+ * unless the read-ahead request is for metadata
+ * (eg, for gfs2 or xfs).
*/
if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) &&
- !(bio->bi_opf & REQ_PRIO))
+ !(bio->bi_opf & (REQ_META|REQ_PRIO)))
goto skip;
if (bio->bi_iter.bi_sector & (c->sb.block_size - 1) ||
@@ -877,7 +878,7 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
}
if (!(bio->bi_opf & REQ_RAHEAD) &&
- !(bio->bi_opf & REQ_PRIO) &&
+ !(bio->bi_opf & (REQ_META|REQ_PRIO)) &&
s->iop.c->gc_stats.in_use < CUTOFF_CACHE_READA)
reada = min_t(sector_t, dc->readahead >> 9,
get_capacity(bio->bi_disk) - bio_end_sector(bio));
--
2.16.4
